1. Field of the Invention
The present invention relates to digital code conversion and more particularly to a digital code compressor for converting linear pulse code modulation signals into companded pulse code modulation signals.
2. Description of the Prior Art
In pulse code modulation (PCM) systems a very common technique to improve dynamic range (signal to quantization noise ratio vs. input signal level) is called companding. A compandor consists of a pair of gain-adjusting circuits which act to maintain a more favorable signal-to-noise ratio over a transmission channel. At the input of such a channel, one unit compresses the amplitude range of the voice signal. The narrowed range enables transmission through a noisy facility at a relatively high level without overloading. At the channel output a complementary unit expands the amplitude range to that of the original input.
Another advantage to companding is that the same dynamic range can be achieved in a companding system with less digits in the PCM codeword than a linear PCM system. This advantage reduces the bandwidth of the signal that is transmitted over the transmission channel. The compression of the signal can be done before coding (analog compression), during coding (by non-linear coders) or after coding (digital compression).
When converting one digital format to another, in some cases there is the need to expand the companded format to a linear format because in the conversion process arithmetic operations must be performed. For example, in the continuous variable slope delta (CVSD) modulation to PCM conversion the companded PCM (CPCM) must be expanded to linear PCM (LPCM) so that the arithmetic algorithms of the CVSD/PCM conversion could be performed.
In the past various code conversion systems have been devised. These prior art systems such as U.S. Pat. No. 3,863,248, issued Jan. 28, 1975, for "A Digital Compressor Expander," utilized arithmetic combined with look-up techniques. The present invention uses the principal of rapid, successive approximation. It makes possible relative simplicity of circuitry.